This invention relates to a bipolar transistor of an integrated circuit (IC) and a method of manufacturing the same.
In an integrated circuit arranged on a semiconductor substrate given an n-type conductivity, such as an n-type silicon substrate, and having a principal surface, an npn bipolar transistor is manufactured along a transistor area surrounded on the principal surface by a field oxide layer which may be made of silicon dioxide and is alternatively called an element isolation layer in the art. The bipolar transistor comprises a highly p-doped base contact region, a highly n-doped emitter region, and a highly n-doped collector region.
In the manner which will later be described in greater detail, the collector region is manufactured as a buried layer under the transistor area. The field oxide layer is selectively formed. As a thin film, the field oxide layer covers the transistor area. Through the thin film, a base hole, an emitter hole, and a collector hole are selectively formed. A part of the thin film remains as a field oxide film in the base hole at its bottom. The base contact region is formed by ion injection of ions of a p-type impurity, such as boron ions, into the semiconductor substrate through the field oxide film. The emitter region is subsequently formed by ion injection. Later, the field oxide film is subjected to chemical removal or etch. Base, emitter, and collector electrodes are formed through the base, the emitter, and the collector holes.
It has been confirmed by the present inventor that the field oxide film is not completely removed by the chemical removal. This is because the field oxide film is highly doped with the p-type impurity during the ion injection to become a film of a compound consisting essentially of silicon and the p-type impurity. This film is herein called a silicon-boron (SiB) film. Due to presence of the silicon-boron film, the base electrode is not brought into an electrically excellent ohmic contact with the base contact region. As a consequence, the bipolar transistor has a forward voltage of higher than 1 volt and a forward voltage gain which is one of high frequency characteristics and is 11 through 13 dB.